This invention relates to the recovery of metals from jarosite-containing materials.
The leaching of certain metals e.g. silver (Ag), lead (Pb) and zinc (Zn) using brine leaching, is well known.(1,2,3,4,5). The ease of solubilising these metals depends on the refractory nature of the material treated.
To improve recoveries from refractory materials by brine leaching, a combined high temperature oxidation process in combination with acidic brine leaching has been proposed.(4) A concentrate containing silver, mostly in sulfide minerals, yielded only 50% Ag dissolution in a FeCl3 brine leach. By leaching the concentrate at temperatures above 100° C. with a high oxygen partial pressure in an acidic NaCl or CaCl2 medium, the Ag recovery was increased to above 95%.
Brine leaching alone is not effective in solubilising metals included in or encapsulated by jarosite or other similar iron hydroxy sulfate compounds since these compounds must first be decomposed.
Decomposition of jarosites in alkaline media is well known. Jarosites produced during pressure leaching of zinc concentrates were decomposed by treating the residues with a lime slurry at 90° C.(7) The following reactions were proposed to describe the reactions for hydronium jarosite, plumbojarosite and argentojarosite respectively:    H3OFe3(SO4)2(OH)6+2Ca(OH)2+H2O→3Fe(OH)3+2CaSO4.2H2O    PbFe6(SO4)4(OH)12+4Ca(OH)2+8H2O→6Fe(OH)3+Pb(OH)2+4CaSO4.2H2O    AgFe3(SO4)2(OH)6+2Ca(OH)2+4H2O→3Fe(OH)3+AgOH+2CaSO4.2H2O
After liberation, Ag was subsequently recoverable by cyanidation.(7) 
Destruction of jarosites produced in pressure leaching at 85° C. to 90° C. using an approximately stoichiometric quantity of lime, followed by cyanidation, improved Ag recoveries from less than 5% to more than 97%.(9) 
NaOH has also been used to facilitate alkaline decomposition of jarosite-type materials before cyanidation.(8) 
Leaching of jarosite-containing materials in an acidic brine medium of CaCl2 at a temperature above the boiling point of the solution and elevated pressures, in the presence of lime or another suitable alkali to maintain the pH between 1.5 and 3.5, has been proposed to recover metals from jarosite(6). Although this method was successful to recover Ag and Pb from jarosites the use of high pressures and temperatures is not always desirable.
It seems obvious that if metal recovery from jarosites is required, an alkaline pretreatment followed by cyanidation is a generally accepted and suitable method. However, cyanide has environmental disadvantages, and in some cases, cyanide consumption is too high for such a process to be economical, particularly in the presence of base metals like Cu, and sulfides.
Also, it is implied that jarosite-containing materials can be pretreated in an alkaline medium to liberate certain metals and that, once the metals are in suitable forms, brine leaching can be used to solubilise them. However, this treatment implies the use of a liquid/solid separation step between the alkaline decomposition stage and the acidic brine solubilisation, and additional process steps and costs. There would be advantages to eliminating this liquid/solid separation, by carrying out the alkaline decomposition in a brine medium followed immediately by acidification to solubilise the required metals. Also, the alkaline decomposition step is shown to be facilitated in a brine solution.